In a variety of types of optical instruments, it is desired that light rays entering an instrument from a range of predetermined directions generally parallel to the axis of the instrument be allowed to reach a focal plane of an optical device or detector. However, it is undesirable for light rays entering the instrument from more extreme angles to be able to reach the focal plane because such out-of-field light rays degrade the desired in-field image. The desired image can be thought of as a signal and the out-of-field light rays reaching the focal plane can be thought of as noise. Thus, performance of an optical instrument is enhanced by increasing the signal-to-noise ratio.
One method for increasing the signal-to-noise ratio is to place an optical shield ahead of the focal plane. An optical shield is designed to deflect or block out-of-field light so that it cannot easily reach the focal plane. Commonly, an optical shield is formed as a tube having a plurality of annular baffles attached to its inner surface and spaced along the length of the tube. One end of the tube admits light through an aperture, and a focal plane is located at the other end of the tube. The baffles are located between the aperture and the focal plane. The baffles extend inwardly toward the center axis of the tube. Frequently, the baffles are coated with an absorptive material such as black paint in order to further reduce unwanted radiation of out-of-field light. Alternatively, the baffles may be reflective on one or both of their surfaces and configured so that out-of-field light is reflected in directions away from the focal plane.
In many cases, insufficient attention is given to light that is scattered in random directions by out-of-field light striking interior surfaces of an optical shield. Because no object can be perfectly absorptive or perfectly reflective, there is always some amount of light scattered in random directions when light strikes an inner surface of an optical shield. If a surface struck by out-of-field light is in view of the focal plane, there inevitably will be some amount of light scattered from that surface to the focal plane. This factor has not been taken into account in the design of some existing optical shields. For example, in some existing optical shields, as illustrated in FIG. 1, light can be reflected off one baffle and strike a rear surface (i.e., the surface facing the focal plane) of the preceding baffle which is in view of the focal plane. Even though this reflected light may not be specularly reflected to the focal plane, some of the light will be scattered to the focal plane.